The Doppler EffectThe Doppler Effect

Ch. 25.9-25.11

Objectives

1. Describe doppler effect

2. Describe bow waves

3. Describe sonic booms

Stationary Source• Bug bobbing at

constant frequency on surface of water

• What do you notice about f & ?

• Bug forms concentric circles.

• Waves encounter point A as frequently as point B.

The Doppler EffectMoving Source

• Bug begins to move across surface @ less than wave speed.

• Which observer (A or B) will encounter a higher frequency of waves?

• Answer: B• Doppler Effect:

– change in FREQUENCY (f) due to the motion of the source

• What about wave speed, does it change?

• NO!!!!

AA B

Doppler EffectMoving Source

• Water waves spread over flat surface, but Sound waves travel in 3-D and expand like balloon

• What do you notice about the waves in front of train compared to those in back when train is stationary? In motion?

• Wave crests ahead of moving source are closer together than behind source

• What does this mean?• Higher frequency in front, lower

frequency in back.– i.e. car/train horn– Demo with string and noise

Doppler Effect

• Bounce radar waves off of moving vehicles

• Compares f of radar waves from gun with the f of reflected waves

RadarRadar

• Q: When a source moves towards you, is there an increase or decrease in wave speed?

• A: Neither! It is the frequency of the wave that undergoes a change, NOT the wave speed.

What happens when the speed of the source is is EQUAL to the speed of waves it produces?

• The waves pile up and superimpose on each other

• This is a barrier wave:– Source and waves are

equal speeds

What happens when the speed of the source is is GREATER than the speed of waves it

produces?• Produces a Bow Wave:

– Speed of source is greater than wave speed

– The wave crests overlap at edges creating a V shape.

Bow Waves

• This can be seen by speedboats

• Increased speed produces a narrower V shape.

• Notice that the boat down below is outrunning the waves it produces

Patterns made by an object moving at successive speeds

Draw these pleaseDraw these please

F-14 Tomcat

Do Planes actually “break the sound barrier?”

Here’s what happens. . .• Overlapping wave crests

disrupt flow of air over wings --> harder to control plane when flying close to speed of sound (761mph)

• But, the barrier is not real• Think of it this way…• Just like a boat travels

faster than speed of water waves…a plane can travel faster than the speed of sound

• Supersonic!!

Shock Waves

• Shock Wave: – produced from overlapping

spheres that form a cone

• Jet “pushes” sound waves in front of it

• Sound waves must “obey” the speed limit--> pile up against each other

• These “piled up” waves are called shock waves

Shock Waves

• Sonic Boom: – the “crack” you hear when the

compressed air of a supersonic object reaches the ground

• Sound waves hit you all at once (not one at a time)

• Boat (one wave at at time) vs. jet (all waves at once)

• A bullet passing overhead produces a “crack” which is a small sonic boom

• Bigger the object --> more air is displaced --> louder the boom

• Sonic Boom Description

Shock Waves

• Shock Waves spread until they reach the ground.

•Crests overlap and encounters listener in a single burst

•So all listeners hear the boom at the same time?

•Shock wave and sonic boom are swept continuously behind aircraft

•Bullets, whips, and towels